NUT Factor / Slip Factor

Dear readers, in continuation with my previous post, Let us discuss about the Coefficient of friction

The basic formula T = K x D x P stated earlier takes these factors into account and provides users with a starting point for establishing an initial target tightening torque. The K factor in this formula is always an estimate. That is the coefficient of friction, frequently referred to as the "Nut factor." / “Slip Factor “

The value of this factor indicates that harder, smoother, and/or slicker bolting surfaces, such as threads and bearing surfaces, require less rotational force (torque) to stretch (tension) a bolt than do softer, rougher, and stickier surfaces

If Surface is not treated Slip factor is 0.20

If Surface is blasted, any loose rust removed, no pitting than Slip factor is 0.50

If Surface is blasted and hot tip galvanised than Slip factor is 0.10

If Surface is blasted and painted than slip factor is 0.30

The most commonly used bolting K factors arc 0.20 for plain finished bolts, 0.22 for zinc plated bolts, and 0.10 for waxed or highly lubricated bolts.

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Bolt Torque Calculations

Dear readers in continuation with my previous post, Let us discuss about the “What torque should be given for the Bolts?”

The only way to properly determine the optimum tightening torque for a given application is to simulate the exact application. This should be done with a tension indicating device like Torque Wrench or Spanner. The widely recognized engineering formula is T= K x D x P

T=Target tighten torque
K= Coefficient of friction (nut factor / Slip Factor), always an estimation in this formula
D = Bolts nominal diameter
P = Clamp Load or Bolt's desired tensile load (generally 75% of yield strength)

Above Three factors affect how much tension occurs when a given amount of tightening torque is applied. The first factor is the bolt's diameter. Naturally M24 bolts take more force to tighten than M16 bolt. The second factor is the bolt's grade. It takes more force to stretch a 10.9 Grade bolt than it does to stretch an 8.8 Grade bolt because of the greater material strength. The third factor is the coefficient of friction often called as nut factor and it is always estimation.

Keep in mind this is only an estimated value. It may provide satisfactory performance, but it also may not. It is extremely important to realize that this tightening value is valid only so long as all of the aspects of the application remain constant. Don’t think if you are giving more torque means it is good for structure / application, because during the torque operation bolt will be stretched. If more stretched than required it may be broken. So each and every application should be evaluated on its own to determine the optimum torque value for each application. If you are not aware please refer the manufacturer recommendations & Technical specification or use above formula with contingency.

Co-Efficient of Friction / Slip factor or Nut Factor details will be followed my Next Post.

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